This invention relates to the field of data storage and retrieval. More particularly, this invention relates to the provision of servo tracks in magnetic disc file units.
In the field of data processing, data is stored in binary digital form on rotating magnetic recording surfaces, such as the surface of a drum or the surface of a disc pack. A disc pack normally includes a plurality of discs fastened to a rotatable hub in a vertically spaced attitude, each disc having a recording surface on both sides. The disc pack is designed for use with a disc file unit having a motor and a spindle connected to the motor output shaft to which the disc pack may be releasably attached, and a bank of transducers mounted on a positioning mechanism for radially moving the transducer head assembly as a unit. Each transducer head in the transducer head assembly is associated to a different one of the disc pack recording surfaces and is adapted to record new data on the associated surface or read data from the surface under control of associated electronic equipment.
Data is recorded on the several surfaces in a circular track format, with each disc surface having a plurality of concentric tracks. Thus, in order to store or retrieve data from a specific track location, an arrangement must be provided for indexing the transducers over the individual tracks.
The conventional technique employed to provide track indexing is a servo surface. The servo surface comprises a dedicated surface of one of the discs in the pack on which a plurality of special tracks, called servo tracks, are prewritten when preparing a disc pack for use. Each servo track has a precisely defined radial width and comprises a series of spaced flux reversals, the spacing increasing with the radial distance of a given servo track from the center of the disc surface. The tracks are arranged in alternate even-odd series, with the sense of the track being determined by the order of the magnetic flux reversals. In an even servo track, the magnetic domains are arranged such that the equivalent electrical signal generated by a read transducer comprises a negative transition followed by a positive transition, while in an odd servo track the equivalent electrical signal comprises a positive transition followed by a negative transition.
In the past, servo tracks have been pre-recorded on a servo surface by using a write transducer with a gap width in the radial direction effectively equal to the desired width of a single track. For example, when writing servo tracks for single density disc packs, the write transducer employed to record the servo tracks has a 5.2 .times. 10.sup.-3 inch radial gap width, which is equal to the radial width of a single density servo track. This arrangement enables a disc pack to operate in a single density mode in which the density of the data tracks on the disc pack data surfaces is approximately 192 tracks per inch. This arrangement for writing servo tracks suffers from the disadvantage that the write transducer must be manufactured to extremely close tolerances which increases the cost of the transducer required.
The single density servo tracks are written by mounting the disc pack on the disc file spindle of a servo track writing unit and actuating the motor to revolve the recording surfaces at a high speed, actuating the write transducer to record a first servo track on the outermost track of the servo surface, radially repositioning the write transducer to the next desired track position, i.e., 5.2 .times. 10.sup.-3 inch radially inwardly while deactivating the write signals supplied from the associated electronic controller, re-energizing the write transducer to record the opposite servo track pattern (odd or even), and continuing this process until the entire servo surface is filled.
In addition to single density servo tracks, a double density servo track arrangement is also employed in the data processing industry. In a double density servo track format, the radial width of each track is 2.7 .times. 10.sup.-3 inch which provides a data track density for the associated data storage surfaces of the disc pack of approximately 370 tracks per inch. In order to record a set of servo tracks in double density format in the known way, it has been the practice to employ a special servo track write transducer whose radial gap width is 2.7 .times. 10.sup.-3 inch, and to employ the same track writing technique as that used for writing single density servo tracks described supra. This necessitates changing the track writing transducer by removing the single density transducer and installing the double density transducer. This requires substantial downtime for the servo track writer unit whenever it is necessary to change from one servo track density format to the other. This arrangement also suffers from the disadvantage that an additional supply of expensive write transducers is required: one for single density purposes, the other for double density purposes.
One approach used to facilitate recording of single and double density servo tracks is disclosed in U.S. Pat. No. 3,875,589. According to this technique, a pair of modular linear positioning units of special construction are provided, each of which can be attached to and removed from an associated servo track writer of special design. One positioning unit is designed to be used for writing single density servo tracks; the other is designed for use in writing double density servo tracks. While effective, this approach requires at least two precision constructed linear positioning units which are relatively expensive to manufacture, and also suffers from the disadvantage that the servo track writer must be shut down in changing from one density format to the other. Other attempts to provide both single and double density track writing capabilities in a single servo track writer have not met with wide success to date.